Leadless solid electrolyte capacitors comprising pressed and sintered powdered valve-metal material achieve high capacitance per volume and are characterized by low weight. In particular, tantalum capacitors are attractive for flat or small products such as, for example, mobile telephones or digital cameras.
In a conventional method for manufacturing a tantalum capacitor with a lead frame, in a first step, a pellet of tantalum powder is pressed and sintered and afterwards singulated forming a tantalum anode slug. Then, the tantalum anode slug is welded to a processing bar. Afterwards, together with the processing bar, the tantalum anode slug is immersed in an anode dielectric formation bath for oxidation by an electrochemical process. Then, several cathode layers containing MnO2 are deposited, before a silver coating is provided over the MnO2 layers. The tantalum slugs are removed from the processing bar and placed on a lead frame, to which anode and cathode are then attached with silver-filled epoxy. After transfer molding during the next step, anode and cathode are electrically plated and the tantalum capacitor is placed into a fixture for voltage conditioning (burn-in).
The above described conventional method partially produces capacitors with poor yields and poor long-term reliability because there is a high likelihood to damage the anodized tantalum anode slug during cathode deposition, lead attachment and transfer molding process steps during which high mechanical stress is applied to the slug. Additionally, the above described process is cost-intensive due to case size-specific overmold tooling and burn-in fixtures, as well as individual part process steps (such as tantalum anode bar welding process, and singulation of the tantalum slug after anodization and cathode deposition process). Such capacitors also have a poor volumetric efficiency because the leaded design itself is volume inefficient (active anode volume/tantalum capacitor volume) due to the lead itself. Such tantalum capacitors further have limited potential for miniaturization as individual part process steps, such as tantalum anode bar welding process and singulation tantalum anode slug after anodization and cathode deposition process, make the miniaturization impractical for capacitors of case size 0603 or smaller.
U.S. Pat. No. 4,599,788 discloses a manufacturing process for a solid electrolyte capacitor by screen printing an array of pads of a tantalum ink composition onto a tantalum substrate, placing a tantalum pellet on one end of said pads, sintering together the assembly of said substrate, pads and pellets, the array is then anodized forming an oxide layer over all the surfaces. Then, a thermally stable polymer is applied in a grid pattern in the spaces along rows and columns, separating the pellet-pad-structures from each other. After that, the cathode is formed by deposition of solid manganese oxide electrolyte and of a conductive layer. A similar process is also described by U.S. Pat. No. 6,849,292. As such known process uses form and size adapted hard tooling to press individual tantalum anode slugs onto a tantalum substrate prior sintering, it is impractical and cost prohibitive to make a tantalum capacitor smaller than 0603 case size with this approach. Additionally, this process requires individual voltage conditioning (burn-in) of each capacitor at the end of the process, which is expensive and provides mechanical stress to each capacitor.
U.S. Pat. No. 3,588,626 refers to a leadless solid electrolyte tantalum capacitor which comprises an anode consisting of a porous tantalum section and a non-porous tantalum section integrally connected with each other, forming a unitary anode. Anodizing the unitary anode provides a singular dielectric layer formed over the surface of the composite anode. Further, a Teflon coating is painted over portions of the dielectric layer formed on the porous tantalum section. Due to the Teflon coating, only the surface of the non-porous tantalum section is covered by a cathode forming a MnO2 layer and an additional conductive layer. Upon removal of the Teflon coating, the surface of the porous section can be used as an anode termination by any desired means. This document describes a method in which the conventional lead (or bar) is replaced with a tantalum pad with same welding process or pressed and sintering process. Therefore, it has the same short coming as the conventional leaded process described above. Additionally, this process requires individual voltage conditioning (burn-in) of each capacitor at the end of the process, which is expensive and provides mechanical stress to each capacitor.
The present invention is directed toward overcoming one or more of the above-mentioned problems.